From: Jezz Bird (jezzbird@hotmail.com)
Date: Tue Feb 12 2008 - 16:31:33 ARST
In a nutshell what Halabi is saying is: if you associate the MED attribute
with an aggregated/summarised route AND supress the 'component' routes, the
MED that you use may not necessarily the best one for ALL the components. The
receiving AS will not know where the components are and in fact as he suggests
they could be scattered throughout the sending AS. The receiving AS will not
have individual MEDs for each of the components and this is when suboptimal
routing may occur.
You will need to understand this before you go to the lab but don't worry
about it because there are bound to be some things that you just won't
understand at first. I would suggest you have a look at Routing TCP/IP Vols I
& II by Jeff Doyle. He explains summarisation/aggregation and BGP brilliantly
!
Regards,
Jezz.> Date: Mon, 11 Feb 2008 21:23:55 -0500> From: guy.jersey@gmail.com> To:
ccielab@groupstudy.com> Subject: can't understand BGP Theory> > From Halabi's
book, Internet Routing Architectures, 2nd edition, page 167:> > *MEDs are
somewhat handicapped by aggregation scenarios in which providers> announce a
given CIDR block from multiple locations in their network and> suppress the
smaller routes from the block. Utilizing MEDs in this scenario> could
potentially result in suboptimal routing because the more-specific> routes of
the CIDR block could be scattered throughout the AS and MEDs> associated with
more-granular routes are no longer available.> > When using MEDs to perform
what's commonly referred to as best-exit routing,> some providers leak the
more-specifics of their CIDR blocks to select peers> to remove the offshoots
introduced by aggregation. The problem with this is> that controlling the
more-specific announcements is sometimes complex, and> failure to do so can
result in some very suboptimal routing situations.> *> I read the above two
paragraphs five times but didn't understand it. Which> of the following is
true:> > a) I have no choice but to understand this stuff, to pass the lab. I
need to> understand *everything* in Halabi's book, period.> b) The lab is
tough but not THAT tough. I can skip certain convoluted> sections of every
topic and still manage to get by.> c) Forget it. I am not going to make it.
MED is a piece of cake; what's so> hard to understand??> d) I need to read
"How to grow gray matter and raise IQ" book before> Halabi's.> > Thing
is....how thoroughly do I need to pound away at theory/reading before> hitting
the equipment, lab scenarios and excercises?> > > thanks, JG> >
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